1. Field of the Invention
This invention relates to a conversion optical system for using interchangeably a finite-corrected optical system with an infinity-corrected optical system, and in particular, to a variable eyepoint height type observation tube (lens-barrel) optical system to which this conversion optical system is applied.
2. Description of Related Art
Optical systems for microscopes are available in two types, one, a finite-corrected optical system forming a primary image directly through an objective lens alone, and the other, an infinity-corrected optical system designed to form the primary image by combining the objective lens with an imaging lens and to bring about a nearly infinite optical beam between the objective lens and the imaging lens. The finite-corrected optical system is characterized by its simple arrangement and compact design, while the infinity-corrected optical system has the feature that, for example, even when various optical elements are introduced into, or removed from, the nearly infinite optical beam, a double image or astigmatism is not produced. Here, the term "nearly infinite optical beam" indicates a case where light emanating from one point is rendered parallel or nearly parallel, for example, by a lens, and the term "convergent beam of light" to be employed later refers to a case where light emanating from one point forms a real image of the point, for example, through a lens.
In recent years, microscopes using infinity-corrected optical systems have become more common. At the same time, there are still many microscope systems using finite-corrected optical systems. It is therefore desirable that optical parts for use in each type of microscope be interchangable.
As an optical system for converting the convergent beam of light emerging from the objective lens of the finite-corrected optical system into the nearly infinite optical beam, a revolver lens provided in an inverted microscope is known. The inverted microscope has the mechanism that, in order to observe a culture sample, the sample is brought to a focus not by moving vertically a stage mounting the sample, but a revolver mounting the objective lens so that a cultured liquid does not spill from a culture vessel. In this type of mechanism, a lens with negative refracting power is incorporated in the revolver, and the convergent beam emerging from the objective lens of the finite-corrected optical system is converted into the nearly infinite optical beam, which is introduced into a microscope body so that an image is formed through an imaging optical system. By constructing such an optical system, even though a distance between the objective lens and the microscope body is changed, the primary image is always formed at a constant position in the microscope body. The negative lens incorporated in the revolver is the revolver lens stated above.
Japanese Patent Preliminary Publication No. Sho 55-155319 discloses an adapter system for using interchangeably the objective lens of the infinity-corrected optical system with that of the finite-corrected optical system. In particular, an adapter for using the objective lens of the finite-corrected optical system in the microscope body with the infinity-corrected optical system has the function of converting the convergent beam emerging from the objective lens of the finite-corrected optical system into the nearly infinite optical beam.
A variable eyepoint height type observation tube, however, is merely provided in accordance with each optical system in such a way that, in an up-right microscope with the finite-corrected optical system, as set forth in Japanese Patent Publication No. Hei 6-97302, an intermediate image is formed once in the midst of the transmission of an image, or in an up-right microscope with the infinity-corrected optical system, as disclosed in Japanese Utility Model Publication No. Hei 4-124218, the intermediate image is not formed.
Inverted and upright microscopes are available. Inverted microscopes having a variable eyepoint height type observation tube provide improved performance, particularly for viewing of multiple specimens in succession. Such a microscope is also convenient for viewing from a half-sitting posture. As a result, the variable eyepoint height type observation tube is desirable, but has the drawbacks that it requires a complicated mechanism and therefore is expensive. Thus, if the observation tube can be used for both an infinity-corrected optical system and a finite-corrected optical system, a considerable savings will be realized.
However, when a variable eyepoint height type observation tube for the infinity-corrected optical system commercially available is used in combination with the Inverted microscope of the finite-corrected optical system, for example, even though an attempt is made to render a nearly parallel beam emerging from the revolver lens incident directly on the variable eyepoint height type observation tube for the infinity-corrected optical system, the distance of the nearly infinite optical beam from the emergence from the revolver lens to the incidence on an imaging lens (telan lens) placed in the variable eyepoint height type observation tube becomes long and the diameter of the beam incident on the imaging lens enlarges. This leads to the problem that the effective diameter of the imaging lens is too small and the periphery of an image will be shaded.
The adapter system disclosed in Sho 55-155319, which is fixed to an objective lens barrel, has the problem that the distance of the nearly infinite optical beam from the emergence from the adapter to the incidence on the imaging lens placed in the variable eyepoint height type observation tube becomes long and thus the periphery of the image is shaded because the effective diameter of the imaging lens is insufficient.
Also, it is conceivable that the variable eyepoint height type observation tube for the up-right microscope with the finite-corrected optical system, such as that disclosed in Hei 6-97302, is applied to the inverted microscope with the finite-corrected optical system. However, since there is a limit to a mechanical tube length, the intermediate image must be formed at least once for the transmission of the image in order to provide sufficient space for a variable eyepoint height mechanism. Where the intermediate image is formed only once, it becomes an inverted image, and as a result, the inverted microscope requiring an erect image to manipulate the specimen cannot be used because its operation property is impaired. Furthermore, when the observation tube is designed so that the intermediate image is formed an even number of times, the entire observation tube becomes very bulky and thus it is hard to use.